1. Field of the Invention
The present invention relates to methodology for diagnosing and treating reading disorders such as dyslexia. More particularly, the present invention relates to methods and apparatus for measuring contrast sensitivity for motion discrimination. The present invention also relates to methods and apparatus for improving contrast sensitivity for motion discrimination. The inventor of the present invention has determined that by improving contrast sensitivity for motion discrimination by practicing the present invention, children who are dyslexic, as well as children with normal reading ability, may improve their reading ability.
2. Description of the Related Art
When a pattern of light falls on the retina, the image is processed within the retina to some extent Ganglion cells of the retina send signals out of the eye to a relay nucleus in the thalamus of the brain. Cells of the thalamus in turn send signals to the visual cortex for further processing. There are two major types of retinal ganglion cells which respectively contact two divisions of cells in the relay nucleus of the thalamus: the parvocellular division and the magnocellular division. Cells in the parvocellular division have small receptive fields and are useful for visual tasks requiring a high degree of acuity. Cells in the magnocellular division, which are about ten-times less numerous than those of the parvocellular division, have large receptive fields and are useful for visual tasks requiring a high degree of movement detection. Cells of the magnocellular division have coarse acuity and high contrast sensitivity.
In view of the above, the vision system of a human may be divided into two visual streams. The first stream is a magnocellular stream which detects the movement of an object. This movement stream has a high sensitivity to low contrast (for example, below 10%), to low luminance, to movement, and has low resolution. The second stream is a parvocellular stream which detects the color, shape, and texture of patterns. This second or acuity steam has low contrast sensitivity and high resolution. The acuity stream is most sensitive to contrasts above about 10%.
The parvocellular and magnocellular cells, either alone or in combination, provide the information used by many different visual cortical pathways (or "streams") which are specialized at performing different perceptual tasks. One such specialized pathway is a visual cortical area called Medial Temporal, or "MT," which is central in the analysis of direction of motion. Most of the signals that drive neurons in area MT derive from neurons in layer 4b of the primary visual cortex, which neurons in turn are primarily supplied by input from the magnocellular cells. (In primates, the primary visual cortex is the only cortical area that receives signals from the retina via neurons in the thalamic relay nucleus.) Direction selectivity is a fundamental characteristic of the magnocellular neurons and is mediated by cells in both layer 4b in the striate cortex and in the MT cortex.
Certain aspects of magnocellular networks, such as direction discrimination and detecting brief patterns, are still developing in all 5 to 9 year old children, when compared to normal adults. Moreover, the immature magnocellular and inhibitory networks of dyslexics confirm the increasing psychophysical, physiological, and anatomical evidence that dyslexics have anomalies in their magnocellular networks, demonstrated by (1) higher contrast thresholds to detect brief patterns, (2) an impaired ability to discriminate both the direction and the velocity of moving patterns, and (3) unstable binocular control and depth localization when compared to age-matched normals. There is substantial evidence that dyslexics have a disordered posterior parietal cortex and corpus callosum, having immature inhibitory networks that severely limit a child's ability to both discriminate direction of movement and read.
Reading is the most important skill that is learned in the first and second grades. Yet there are no standardized ways to evaluate or to teach reading. A natural assumption is that reading relies on the higher resolution pattern system evaluated by measuring an observer's visual acuity and color discrimination ability. It is generally believed that movement discrimination is involved in reading solely as a means of directing eye movements, coordinating each saccade so that letter recognition can be conveyed by the portion of the vision system which has a higher resolution. It is intriguing that differences between children with reading problems (e.g., those who are dyslexic) and children with normal reading ability were revealed only by tests of the cortical movement system. On the other hand, tests of the pattern system, such as visual acuity using long duration patterns, revealed no differences between children with normal reading and children with reading problems. However, a recent study questions whether dyslexic children show a temporal processing deficit, and another study concludes that the contrast sensitivity functions (CSFs) of dyslexic children are unrelated to their reading ability.
A natural assumption in the art is that reading relies on the high-resolution acuity system. The acuity system may be evaluated by measuring the visual acuity of a subject, which is measured by an index of 20/20, 20/40, and so on as known in the art. Conventional wisdom in the art teaches that dyslexia, which may be defined as a difficulty in reading in a child of normal intelligence and an adult-level acuity (i.e., 20/20), is explained as a difficulty in decoding words on a page that are readily seen.
One approach used to remediate dyslexia involves training the child to engage in novel, small-scale hand-eye coordination tasks like drawing, painting, and modeling, coupled with word identification, for 5 hours per week over 8 months. This approach improved reading at least one grade level. The mechanism for this improvement is unknown.